Organic light-emitting diode (OLED) technology incorporates organic luminescent materials that, when sandwiched between electrodes and subjected to a DC electric current, produce intense light of a variety of colors. These OLED structures can be combined into the picture elements or pixels that comprise a display. OLEDs are also useful in a variety of applications as discrete light-emitting devices, or as the active element of light-emitting arrays or displays, such as flat-panel displays in watches, telephones, laptop computers, pagers, cellular phones, calculators, and the like. To date, the use of light-emitting arrays or displays has been largely limited to small-screen applications such as those mentioned above.
Demands for large-screen display applications possessing higher quality and higher resolution has led the industry to turn to alternative display technologies that replace older LED and liquid crystal displays (LCDs). For example, LCDs fail to provide the bright, high light output, larger viewing angles and speed requirements that the large-screen display market demands. By contrast, OLED technology promises bright, vivid colors in high resolution and at wider viewing angles. However, the use of OLED technology in large-screen display applications, such as outdoor or indoor stadium displays, large marketing advertisement displays, and mass-public informational displays, is still in the development stage.
Several technical challenges exist relating to the use of OLED technology in a large-screen application. One such challenge is that OLED displays are expected to offer a wide dynamic range of colors, contrast and light intensity depending on various external environmental factors including ambient light, humidity and temperature. For example, outdoor displays are required to produce more white color contrast during the day and more black color contrast at night. Additionally, light output must be greater in bright sunlight and lower during darker, inclement weather conditions. The intensity of the light emission produced by an OLED device is directly proportional to the amount of current driving the device. Therefore, the more light output needed, the more current is fed to the pixel. Accordingly, less light emission is achieved by limiting the current to the OLED device.
A pixel, by definition, is a single point or unit of programmable color in a graphic image. However, a pixel may include an arrangement of sub-pixels, for example red, green and blue sub-pixels. It is known that such sub-pixels can be driven by a drive circuit having a common cathode configuration. According to a new technology, also a common anode configuration can be applied. These configurations refer to whether the three sub-pixels are addressed via a common cathode line or via a common anode line, respectively. Accordingly, in the common cathode configuration, the cathodes of the three sub-pixels are electrically connected and addressed in common. In the common anode configuration, the anodes of the three sub-pixels are electrically connected and addressed in common.
In the known common cathode drive circuit, a current source is arranged between each individual anode and a positive power supply, while the cathodes are electrically connected in common to ground. Consequently, the current and voltage are not independent of one another, thus small voltage variations result in fairly large current variations, having the further consequence of light output variations. Furthermore, in the common cathode configuration the constant current source is referenced to the positive power supply, so any small voltage variation results in a current variation. For these reasons, the common cathode configuration makes precise control of the light emission, which is dependent upon precise current control, more difficult.
By contrast, in an anode drive circuit, a current source is arranged between each individual cathode and ground, while the anodes are electrically connected in common to the positive power supply. As a result, the current and voltage are completely independent of one another; thus, small voltage variations do not result in current variations, thereby eliminating the further consequence of light output variations. Furthermore, in the common anode configuration the constant current source is referenced to ground, which does not vary, thereby eliminating any current variations due to its reference. For these reasons, the common anode configuration lends itself to precise control of the light emission needed in a large-screen display application.
Another consideration is that a common anode design requires NPN transistor design while common cathode design requires PNP transistor design. NPN transistors are smaller and faster than PNP transistors, which employ holes to carry the electric current as opposed to electrons. The electron carriers of the NPN transistors are smaller and much more mobile than their PNP counterparts. As a result, PNP transistors are 30-50% more costly than NPN transistors to manufacture because they require a larger quantity of materials for production.
An example of a pixel drive circuit is found in reference to U.S. Pat. No. 6,512,334, entitled, “Organic electroluminescence matrix-type single-pixel drivers.” This patent describes an organic electroluminescence (OEL) matrix-type single-pixel driver that comprises an OEL device, a first transistor and a second transistor. The first transistor and the second transistor form a complementary structure so that when the data line uses the first transistor to drive an OLED device, the second transistor is in the OFF state, causing no power consumption. When the data line is in the LOW state, the first transistor is in the OFF state. The second transistor is in a sub-threshold state after getting rid of extra charges.
Although the control circuit described in U.S. Pat. No. 6,512,334 employs a switching mechanism to control anode voltages, it does not employ a common anode design, nor does it provide a means for incorporating smaller, faster and less expensive components. Furthermore, the drive circuit described in U.S. Pat. No. 6,512,334 provides only voltage control to each individual pixel in the matrix display and thus provides no means for the high currents necessary to produce high light output. Finally, the drive circuit of U.S. Pat. No. 6,512,334 does not provide a means for varying the amount of light output or controlling contrast in a high resolution passive matrix display.